The present invention relates to telecommunications in general, and, more particularly, to a technique for handing off a wireless terminal from one base station to another as the wireless terminal moves within a wireless telecommunications system.
FIG. 1 depicts a schematic diagram of a portion of a wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals 101-1 through 101-3) that are situated within a geographic region. The heart of a wireless telecommunications system is a wireless switching center (xe2x80x9cWSCxe2x80x9d), which also may be known as a mobile switching center or mobile telephone switching office. Typically, a wireless switching center (e.g., WSC 120) is connected to a plurality of base stations (e.g., base stations 103-1 through 103-5) that are dispersed throughout the geographic region serviced by the system and to the local and long-distance telephone and data networks (e.g., local-office 130, local-office 138 and toll-office 140). A wireless switching center is responsible for, among other things, establishing and maintaining a call between a first wireless terminal and a second wireless terminal or, alternatively, between a wireless terminal and a wireline terminal (e.g., wireline terminal 150), which is connected to the system via the local and/or long-distance networks.
The geographic region serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called xe2x80x9ccells.xe2x80x9d As depicted in FIG. 1, each cell is schematically represented by a hexagon. In practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with the wireless switching center.
For example, when a user of wireless terminal 101-1 desires to transmit information to a user of wireless terminal 101-2, wireless terminal 101-1 transmits a data message bearing the user""s information to base station 103-1. The data message is then relayed by base station 103-1 to wireless switching center 120 via wireline 102-1. Because wireless terminal 101-2 is in the cell serviced by base station 103-1, wireless switching center 120 returns the data message back to base station 103-1, which relays it to wireless terminal 101-2.
Typically, a wireless terminal communicates with the base station that is nearest to it. But because a wireless terminal can move from cell to cell, the base station that the wireless terminal is nearest to can change. Therefore, a wireless terminal usually communicates with different base stations as it moves from cell to cell. For example, if wireless terminal 101-1 exits from the cell serviced by base station 103-1 and moves into the cell serviced by base station 103-2, wireless terminal 101-1 ends communication with base station 103-1 and begins communication with base station 103-2. The process in which a wireless terminal ends communication with one base station and begins communication with another is, for the purposes of this specification, defined as a xe2x80x9chand off.xe2x80x9d
Although there are several techniques in the prior art for performing a hand off, all of the techniques are similar in that they attempt to orchestrate the process so deftly that there is no perceptible interruption in service.
One technique in the prior art for performing a hand is known as xe2x80x9chard hand off.xe2x80x9d Before a hard hand off, a wireless terminal uses one pair of communications channels (one for transmitting, the other for receiving) for communicating with a first base station. At the moment when the wireless terminal exits the cell serviced by the first base station and enters the cell serviced by a second base station, the wireless terminal re-tunes its radio from the first pair of communications channels to a second pair of communications channels for communicating with a second base station. Furthermore, at the same moment that the wireless terminal re-tunes its radio, the wireless switching center stops using the first base station for communication with the wireless terminal and begins using the second base station.
For example, FIG. 2 depicts a block of spectrum that has been partitioned into four frequency bands, channel #1 through channel #4. If wireless switching center 120 assigns channels #1 and #2 to base station 103-1 and channels #3 and #4 to base station 103-2, then base station 103-1 could transmit in channel #1 and receive in channel #2, and base station 103-2 could transmit in channel #3 and receive in channel #4. Because neither base station 103-1 nor 103-2 use the same channels for communicating, there is no co-channel interference between the base stations.
To continue with the example, assume that when wireless terminal 101-1 is in the cell serviced by base station 103-1, wireless terminal 101-1 uses channels #1 and #2 to communicate with base station 103-1. Thereafter, when wireless terminal 101-1 exits the cell serviced by base station 103-1 and moves into the cell serviced by base station 103-2, wireless terminal 101-1 ends communications in channels #1 and #2 and begins communication in channels #3 and #4 with base station 103-4. Furthermore, at the moment that wireless terminal 101-1 re-tunes from channels #1 and #2 to channels #3 and #4, wireless switching center 120 stops routing the signals intended for wireless terminal 101-1 to base station 103-1 and begins routing the signals to base station 103-2. And still furthermore, at the moment that wireless terminal 101-1 re-tunes, wireless switching center 120 expects the signals from wireless terminal 101-1 to be received from base station 103-2, rather than from base station 103-1.
Hard hand off is advantageous in that it can be used with all multiplexing technologies (e.g., frequency-division multiplexing (hereinafter xe2x80x9cFDMxe2x80x9d), time-division multiplexing (hereinafter xe2x80x9cTDMxe2x80x9d), code-division multiplexing (hereinafter xe2x80x9cCDMxe2x80x9d), etc.) and all access technologies (e.g., frequency-division multiple access (hereinafter xe2x80x9cFDMAxe2x80x9d), time-division multiplex access (hereinafter xe2x80x9cTDMAxe2x80x9d), code-division multiple access (hereinafter xe2x80x9cCDMAxe2x80x9d), etc.). Hard hand off is further advantageous in that it is a rather simple process to implement.
Hard hand off is, however, disadvantageous in that it is a fragile process. If the re-tuning of wireless terminal 101-1 is not precisely synchronized with the switching from base station 103-1 to base station 103-2, then there could be an interruption in service. Such a lapse is not only an inconvenience, but also creates the perception that the quality of service offered by the wireless telecommunications system is inferior. This, in turn, causes customers to use competing services and/or to use less of the service than they might otherwise. The net result is that the owner of the wireless telecommunications system loses revenue because hand off in the prior art is such a fragile process.
A second technique in the prior art for handing off a wireless terminal is known as xe2x80x9csoft hand off.xe2x80x9d Before a soft hand off, a wireless terminal uses one pair of communications channels (one for transmitting, the other for receiving) for communicating with a first base station. Before the wireless terminal exits the cell serviced by the first base station and enters the cell serviced by the second base station, the wireless terminal begins communications with the second base station on a second pair of channels. Thereafter, the wireless terminal communicates with both base stations on different pairs of channels until the wireless terminal ends communication with the first base station.
The salient difference between hard hand off and soft hand off is that with soft hand off there is no single moment when the wireless terminal stops using one base station and begins using the second. Rather, with soft hand off there is a significant interval (e.g., a few seconds or more) when the wireless terminal communicates with two base stations at the same time. In contrast, with hard hand off, there is no time when the wireless terminal communications with two base stations at the same time.
Soft hand off is advantageous in that it can be used with all multiplexing and access technologies. Soft hand off is further advantageous in that it is robust. The interim of overlapping communications creates redundancy and, therefore, it is less likely that there will be an interruption in service.
Soft hand off is disadvantageous, however, in three respects. First a wireless terminal capable of soft hand off must comprise two transceivers (one for communicating with each base station) and the circuitry needed to coordinate the two transceivers. This increases the cost of the wireless terminal considerably.
Second, a wireless switching center capable of soft hand off must comprise the circuitry needed to communicate with a wireless terminal through two independent channels. This increases the cost of the wireless switching center.
Third, soft hand off requires that a wireless terminal occasionally uses two sets of communications channels, which precludes the use of one of them by another wireless terminal. This lowers the traffic capacity of the telecommunications system.
Therefore, the need exits for a technique for handing off a wireless terminal from one base station to another that is robust and yet more economical to implement than soft hand off.
The present invention is a wireless telecommunications system that is capable of handing off a wireless terminal without some of the costs and disadvantages of techniques for hand off in the prior art. In particular, the present invention performs a hand off that is as robust as soft hand off, yet costs substantially less. In particular, a wireless terminal in accordance with the present invention needs only one transceiver and uses only one set of communications channels. Furthermore, the present invention is suitable for use with all multiplexing and access technologies and all current air interface standards (e.g., IS-41, IS-54, GSM, etc.).
The present invention is most easily understood by comparing and contrasting its salient characteristics with hard hand off in the prior art. In accordance with hard hand off, a wireless switching center, both base stations and a wireless terminal are all involved in the hand off process. In contrast, a hand off in accordance with the present invention can be performed without by just the wireless switching center and base stationsxe2x80x94the wireless terminal can, but need not, participate and need not even know that a hand off has occurred.
In accordance with hard hand off, the wireless terminal re-tunes its transceiver. Instead, the present invention allows the wireless terminal to use the same communications channels both before and after the hand off.
In other words, in a hard hand off each base station is associated with a different pair of communications channels. Before the hand off, the wireless terminal uses one pair of communications channels for communications with a first base station. To accomplish the hand off, the wireless terminal re-tunes its transceiver to use a second pair of communications channels for communicating with a second base station. The salient characteristic of the prior art is that different pairs of communications channels are used for communicating with different base stations and the wireless terminal is responsible for re-tuning from the first pair to the second pair.
In accordance with the illustrative embodiment of the present invention, communications channels are not associated with a base station. Instead, each pair of communications channels are associated with a wireless terminal and the wireless terminal uses that pair of communications channels both before and after a hand off. Before the hand off, the wireless terminal uses the pair of communications channels for communicating with a first base station. To accomplish the hand off to a second base station, the first base station stops using the communications channels at the same time that the second base station starts using them.
The result is that although the wireless switching center must switch from using the first base station to the second base station, as with hard hand off, there is no re-tuning of the wireless terminal that it must be coordinated with. This permits the hand off to be very robust.